Measuring brain docosahexaenoic acid turnover as a marker of metabolic consumption

Docosahexaenoic acid (DHA) is a 22‑carbon omega-3 polyunsaturated fatty acid (n-3 PUFA) with six double bonds, commonly denoted as 22:6n-3. Among dietary n-3 PUFA, DHA is uniquely accreted in tissue phospholipid (PL) membranes, especially being concentrated in mitochondria dense tissues including the brain, retina, and spermatozoa (Brenna & Diau, 2007; Esmaeili, Shahverdi, Moghadasian, & Alizadeh, 2015; Fu et al., 2019; Svennerholm, 1964). Whole human brains contain 3.47 g of DHA which is the most abundant n-3 PUFA at 7.7% of total brain fatty acids (Lacombe et al., 2023). High unsaturation of DHA regulates membrane fluidity and the structural assembly of lipid raft and enzyme complexes; thereby affecting downstream signal transduction (Calder, 2016). In addition to structural roles, DHA de-esterified from PL is a precursor to an increasing superfamily of bioactive metabolites that regulate neurite growth, synaptogenesis, inflammation, and oxidative stress (Kim & Spector, 2018; Ponce et al., 2022; Tiberi & Chiurchiu, 2021). Unsurprisingly, reductions in blood and/or brain DHA have been observed in patients with neurodegenerative and psychiatric diseases (Bazinet & Laye, 2014; Lacombe, Chouinard-Watkins, & Bazinet, 2018; Liao et al., 2019). Although the effects of DHA on disease pathology and correlations to pathological phenotype are relatively consistent in preclinical models and observational studies, respectively, randomized clinical trials (RCT) and meta-analyses report largely null or inconsistent effects on brain related outcomes with pure DHA supplementation especially when predetermined primary outcomes are considered (AlAmmar et al., 2021; Andriambelo, Stiffel, Roke, & Plourde, 2023; Gould, Roberts, & Makrides, 2021; Liao et al., 2019; Miller et al., 2022; Saber et al., 2017; Sala-Vila et al., 2021; Sinclair, 2019; Stonehouse et al., 2013; Trepanier, Hopperton, Orr, & Bazinet, 2016; Yurko-Mauro, Alexander, & Van Elswyk, 2015).

DHA has been considered to be a conditionally essential nutrient since it may be synthesized from its 18‑carbon dietary precursor, α-linolenic acid (ALA; 18:3n-3) (Cunnane, 2003). Nevertheless, the rate of synthesis of DHA from ALA in the liver and its capacity to maintain brain DHA levels remains a highly debated topic (Domenichiello, Kitson, & Bazinet, 2015; Metherel & Bazinet, 2019; Sinclair, 2019). In contrast, the rate of DHA synthesis from ALA in the brain is much lower than DHA uptake from circulation (Demar Jr., Ma, Chang, Bell, & Rapoport, 2005). Therefore, brain DHA PL levels are maintained via uptake and incorporation of circulating DHA. While brain PL levels of DHA are affected by dietary intake of preformed DHA, especially when ALA is lacking, the plateau of brain DHA levels appear to be tightly regulated under normal conditions (Chen et al., 2020; Giuliano, Lacombe, Hopperton, & Bazinet, 2018; Hsieh & Brenna, 2009; Orr, Tong, Kang, Ma, & Bazinet, 2010). PL membrane remodeling is a rapid and active process where fatty acids, including DHA, are activated by CoA esterification by acyl-CoA synthetases, esterified by acyltransferases, de-esterified by phospholipases, and re-esterified to the PL membrane, and is commonly referred to as the Land's Cycle (O'Donnell, 2022). Therefore, while brain DHA concentrations and composition may appear relatively stagnant, the kinetic rates of DHA uptake, incorporation, turnover, and loss in the brain may be independently regulated and significant.

The methods to measure brain DHA uptake and turnover have been developed by Rapoport and colleagues over 30 years ago and are briefly summarized below (Robinson et al., 1992). Multiple lipid pools in the blood may supply DHA to the brain. As the predominant supplier of brain DHA, albumin-bound unesterified DHA is rapidly taken up by the brain via passive diffusion (Fig. 1) (Lacombe et al., 2018). Uptake of DHA is defined by two kinetic terms: k* (incorporation coefficient) and Jin (rate of incorporation from the plasma unesterified pool). While k* quantifies the incorporation of labeled fatty acids from plasma to stable brain lipid pools, Jin extends k* to quantify the net rate of incorporation of unlabeled fatty unesterified fatty acids to stable brain lipid pools. Albeit a relatively small contribution to total brain DHA uptake, esterified forms of DHA such as lysophosphatidylcholine-DHA (LPC-DHA), have relatively high brain-body partition coefficients due to their longer circulating half-life as compared to unesterified DHA (Bazinet, Bernoud-Hubac, & Lagarde, 2019). Upon uptake, DHA is activated by long-chain acyl-CoA synthetase 6 (ACSL6) initiating the esterification of DHA-CoA to brain PL (Chouinard-Watkins & Bazinet, 2018; Fernandez et al., 2018). DHA-CoA esterification to brain PL is catalyzed by AGPAT4/lysoPA acyltransferase δ (LPAATδ) and lysophosphatidylethanolamine acyltransferase 2 (LPEAT2) both of which demonstrated activity with DHA-CoA (Eto, Shindou, & Shimizu, 2014; Eto, Shindou, Yamamoto, Tamura-Nakano, & Shimizu, 2020; Kitson, Stark, & Duncan, 2012). Esterification of plasma DHA to brain PL is denoted by the Jin; while the net rate of DHA esterification is represented by JFA, which is derived experimentally by adjusting Jin for dilution in the DHA-CoA pool (Fig. 1) (Green, Orr, & Bazinet, 2008). During Lands' recycling, DHA is de-esterified by phospholipase A2 (PLA2) from brain PL where 96–98% of the released DHA is re-esterified to PL, and the remainder is lost to either mitochondrial/peroxisomal β-oxidation, autoxidation, or enzymatic synthesis of bioactive oxylipins, endocannabinoids, and N-docosahexaenoyl amides, collectively characterized by the rate of loss, Jout (Rapoport, 2001, Rapoport, 2003). Since mitochondrial β-oxidation of DHA is low, DHA loss in the brain may reflect synthesis of bioactive metabolites (Chen et al., 2013; Chen, Liu, Ouellet, Calon, & Bazinet, 2009).

In this review, we will summarize the loss of brain DHA to the ever-expansive DHA derived metabolome with a specific focus on bioactive metabolites detected and quantified in the brain of murine models and human post-mortem samples. Then, we will discuss the methodologies to kinetically trace the incorporation (Jin), turnover (JFA), and loss (Jout) of plasma DHA to stable brain lipid pools in murine and human brains using radiolabeled (14C and 11C), and natural abundance of 13C enriched DHA kinetic modeling.

留言 (0)

沒有登入
gif